1. Field of the Invention
The invention concerns an apparatus for measuring a rotary member unbalance as is known from DE 41 24 285 A1.
2. State of the Art
The known apparatus has a main shaft which is mounted rotatably in a machine housing and to which a rotary member to be balanced, in particular a motor vehicle wheel, can be fixed. The main shaft is driven by a single-phase ac motor. It has a single-phase main winding and an auxiliary winding which is electrically shifted in respect of phase, in particular by 90xc2x0. The pendulum oscillations which occur in the case of a single-phase ac motor, in particular when operating in an approximately load-free condition, are reduced by an oscillator circuit into which the auxiliary winding is incorporated. In contrast to apparatuses in which the drive is switched off or disconnected after the measuring speed is reached, it is possible even without a wheel to check or measure the unbalance of the main shaft and/or the clamping means. In addition in adjusting the apparatus the measuring time and the measuring accuracy are not limited.
An object of the invention is to provide an apparatus of the kind set forth in the opening part of this specification, in which pendulum oscillations are suppressed, in particular in the approximately load-free mode of operation, over a wide range of preselectable measuring speeds.
In accordance with the invention that object is attained. The invention comprises an apparatus for measuring a rotary member unbalance comprising a main shaft which is mounted rotatably in a machine housing and to which the rotary member to be measured can be fixed, a single-phase ac motor for driving the main shaft with a single-phase main winding and an auxiliary winding which is displaced in phase relationship for driving the main shaft up to a measurement speed and a control device which adjusts the supply current, wherein the control device has a device which when the measurement speed is reached reduces the supply current supplied to both windings of the motor so that the motor produces a reduced torque which substantially compensates for a speed drop caused by friction and air resistance.
Preferably, when the measuring speed is reached, the torque of the motor is not completely switched off but switched over to a first low torque. The magnitude of that first load torque is such that a reduction in speed which is caused by friction, in particular in the drive, or by air resistance of the rotating rotary member, in particular a motor vehicle wheel, is compensated. In that respect the low torque is of such a magnitude that there is a slight rise in speed until an upper speed limit is reached. When the upper speed limit is reached the torque is reduced to a second, somewhat lower value at which the speed slowly falls, or the torque is switched off and, when a lower speed limit is reached, the first reduced torque is switched on again. In that way the measuring speed can be maintained as long as may be desired. The speed of the main shaft can be detected by way of an incremental generator or rotary speed sensor designed in known manner and by means of two-step control it is possible to implement the appropriate setting of the motor current to produce the reduced torque in order to maintain the measuring speed between the upper limit and the lower limit. In that respect, the two-step control provides that the motor current is adjusted to two suitable values which provide for creation of the first and second reduced torque values by the motor in dependence on the upper and lower speed limits. After the measuring run the main shaft is braked to a stop in known manner by reversal of the maximum motor torque. The motor is switched into a current-less condition just before the stopped condition is reached.
The reduction in the torque produced by the motor when the measuring speed is reached can be effected in various ways. For example an electrical resistor which is connected into the supply line of the main winding and the auxiliary winding or can be achieved by a reduction in the voltage in the supply line. The resistor can be in the form of a reactance or an ohmic resistor. Preferably an inductive resistor in the form of a choke is used. That resistor has a low level of inherent heating and provides for sinusoidal current draw, even if harmonics are superimposed on the mains. By means of a relay, it is possible to bridge over the resistor or to apply the full supply voltage again, so that it is possible to switch between reduced and full motor torque. In addition an ac voltage controller, in particular semiconductor-based, which is controlled by pulse width modulation, can be used. A reduced torque can also be achieved by switching in additional motor windings.
The invention can use a four-pole single-phase ac motor which is fed from the mains, in particular a capacitor motor. The torque generated by the motor is transmitted by way of a suitable transmission, for example by way of a belt drive, to the main shaft of the balancing machine. That can be accelerated for example in the limits of between 80 and 210 rpm to a preselectable measuring speed and maintained there, in which case pendulum oscillations of the motor upon reaching the measuring speed are avoided by virtue of the reduction in torque and the above-discussed two-step control. That results in particular from the fact that the motor delivers only a low level of torque and possible pendulum oscillations are reduced, in relation to the reduction in the torque. The mains frequency has practically no influence on the measuring speed. Current consumption and thus the rise in temperature of the motor are reduced as the motor, in particular when using an inductive resistor, always involves a sinusoidal current draw in the current path of the main winding.
Braking of the main shaft is possible by reversing the motor torque so that there is no need for a separate operating brake. In this case it is possible to effect braking into the desired balancing position in the one balancing plane and then turning the rotary member into the balancing position in the other balancing plane. Preferably identical windings are used for the main and auxiliary windings, for that purpose. Measurement of the unbalance of the main shaft and/or the clamping means without a wheel or flywheel mass is possible, with any measuring period. By virtue of the two-point control with a reduced torque, the measuring speed can be maintained for any desired period within close limits (upper and lower speed limits).